With the widespread use of portable color-display devices, there has been a rapidly growing demand for printing in a simple manner photographs or documents that are captured or prepared using such devices in color. This may be realized by a color printing method such as electrophotography, an ink-jet method, or thermal transfer recording. In particular, thermal transfer recording is a suitable method for realizing printing in a simple manner regardless of circumstances, because it enables printing to be performed using a dry process and allows reduced printer size, which increases printer portability. In addition, dyes are used as colorants in thermal transfer recording. This enables the density of images to be controlled by changing the density and gradations of the colorants, which increases the vividness of the images and enables a high color-reproduction capability to be achieved.
Thermal transfer recording is an image-forming method in which a thermal transfer sheet constituted by a sheet-like base material and a colorant layer disposed on the base material, the colorant layer including thermally migratable coloring matter, is superimposed on an image-receiving sheet including a coloring-matter-receiving layer disposed on the surface thereof, and the coloring matter included in the thermal transfer sheet is transferred to the image-receiving sheet by heating the thermal transfer sheet in order to perform recording. In thermal transfer recording, colorants included in the transfer sheet or in an ink composition included in the transfer sheet are critical materials that affect the speed of transfer recording and the quality and preservation stability of the recorded images.
There have been reported studies (Japanese Patent Laid-Open No. 5-262056, Japanese Patent Laid-Open No. 7-096675, and Japanese Patent Laid-Open No. 8-011450) in which yellow dyes, magenta dyes, and cyan dyes are used as colorants in the above-described thermal transfer recording. Images are formed using a yellow ink sheet, a magenta ink sheet, and a cyan ink sheet, which are prepared using the yellow dyes, the magenta dyes, and the cyan dyes, respectively. Therefore, black can be reproduced by mixing the three colors together (process black). In order to produce high-quality process black, it is necessary to use a combination of colorants which achieves uniform absorption over the entire visible spectrum. However, the higher the image vividness and color-reproduction capability required, the higher the chroma of the dyes needs to be, with such dyes exhibiting a steep and narrow absorption spectrum. As a result, wavelength components that are not absorbed may remain when the colors are mixed together, which disadvantageously hinders high-quality black from being produced.